1. Field of the Invention
The present invention relates generally to rheological materials, and more particularly, to methods and devices that utilize rheological materials, particularly, medical methods and devices.
2. Prior Art
Rheological materials refer to materials that change their state from a freely flowing liquid state to a stiffened near-solid state in response to an external stimulus. There are three basic types of rheological materials, they are electro-rheological (ER) fluids which change state in response to an applied electrical field, magneto-rheological (MR) fluids (also referred to as an MR suspension) which change state in response to an applied magnetic field, and liquid evacuation materials which change state in response to the evacuation of liquid from the material. The latter two rheological materials are referred to herein as rheological fluids.
Electro-rheological (ER) fluids are suspensions consisting of dielectric particles of size 0.1-100 m and dielectric base fluid. Since the dielectric constant of the suspension's particles differs from the dielectric constant of the base fluid, external electric field polarizes the particles. These polarized particles interact and form chain-like or even lattice-like organized structures. Simultaneously the rheological properties of the suspension change effectively, e.g. the effective viscosity increases dramatically.
ER fluids react rapidly to the applied field. The response time of electro-rheological fluids is of the order of 1-10 ms, which in principle enables the use of these liquids in such applications as electrically controlled clutches, valves and active damping devices.
ER suspensions also have a magnetic analog consisting of ferromagnetic particles and a base liquid. As the viscosity of the electro-rheological liquid can be controlled with the electric field strength, the viscosity of magneto-rheological (MR) fluid is sensitive to a magnetic field.
MR fluids are suspensions of micron-sized, magnetizable particles in a carrier fluid. Normally, MR fluids are free-flowing liquids having a consistency similar to that of motor oil. However, when a magnetic field is applied, their rheology changes, virtually instantly, to a near-solid consistency. Altering the strength of an applied magnetic field will precisely and proportionally control the consistency or yield strength of MR fluids, which behave as Bingham solids when in the presence of a magnetic field.
As shown in FIGS. 1a, 1b, and 1c, ER and MR fluids can be used in valve mode (FIG. 1a) with fluid flowing through an orifice, in a shear mode (FIG. 1B) with the fluid flowing between two surfaces which move relative to each other, or in squeeze film mode (FIG. 1c) where the fluid is compressed between two surfaces. In the absence of a magnetic field applied across a gap in which the fluid occupies, the fluid flows freely or allows free movement. In FIGS. 1a, 1b, and 1c, H denotes the applied magnetic field, F denotes the applied force, f denotes the fluid flow, and d denotes the displacement. Furthermore, reference numerals 100 and 102 denote first and second plates, respectively, while reference numeral 104 denotes a MR fluid disposed between the plates 100, 102. Examples of devices that utilize the valve mode include servo-valves, dampers, and shock absorbers. Examples of devices that utilize shear mode include clutches, brakes, and chucking and locking devices. Squeeze mode is typically utilized in applications having high forces and low motion. Although, FIGS. 1a, 1b, and 1c, are shown with regard to MR fluids, the modes illustrated therein are equally applicable to other rheological materials.
Upon application of a magnetic field, the particles align like chains with the direction of the field. The formation of these particle chains restricts the movement of the fluid within the gap since the fluid's yield strength is increased. Altering the inter-particle attraction by increasing or decreasing the strength of the field permits continuous control of the fluid's rheological properties and hence the damping or clutch or braking force.
Magneto-rheological or MR fluids are essentially suspensions of micron-sized, magnetizable particles in oil. Under normal conditions, a rheological fluid is a free-flowing liquid with a consistency similar to that of motor oil. Exposure to a magnetic field, however, can transform the fluid into a near-solid in milliseconds. Just as quickly, the fluid can be returned to its liquid state with the removal of the field. The degree of change in an MR fluid is proportional to the magnitude of the applied magnetic field. When subjected to the field, rheological fluids actually develop a yield strength and behave as Bingham solids. The change can appear as a very large change in effective viscosity. Iron particles in rheological MR fluids instantly form a chain when exposed to a magnetic field, changing the fluid from free-flowing to near solid.
MR fluids are similar to ER fluids but are 20-50 times stronger. They can also be operated directly from low-voltage power supplies and are far less sensitive to contaminants and extremes in temperature. Applied to a variety of devices, MR fluids can provide flexible control capabilities in designs that are far less complicated and more reliable than conventional electro-mechanical products.
Fluid evacuation materials are generally disposed in a flexible bladder that is connected to an evacuation means, such as a vacuum pump by one or more vacuum ports. The flexible bladder is substantially flexible when suction is not applied to the ports and is substantially rigid when vacuum is applied to the ports. The flexible bladder has beads suspended in a fluid, when suction is applied, the volume in the flexible bladder is collapsed, thereby urging the beads into a closer relationship and increasing the density thereof. The beads can take any shape, and can be shaped similarly or dissimilarly. In this state, the collapsed flexible bladder is substantially inflexible, resists bending, and retains a stiffened position. The flexible bladder can include a mesh and/or a plurality of cells to amplify the stiffening effect of the evacuation.
The following publications are recommended for a more thorough review of rheological materials and principles, Carlson, What Makes a Good MR Fluid, 8th International Conference ON Electrorheological (ER) Fluids and Magneto-rheological (MR) Suspensions, Nice Jul. 9-13, 2001; Henrie et al., Variable Compliance via Magneto-Rheological Materials, Proceedings of the 43rd International Symposium, Anaheim, Calif., pp. 431-443, Jun. 1998; and Nakano et al., Electro-Rheological Fluids, Magneto-Rheological Suspensions and their Applications, Proceedings for the 6th International Conference, Yonezawa, Japan, Jul. 22-25, 1997, all of which are hereby incorporated by their reference.